Castings are often required to have holes and recesses of various sizes and shapes. These impressions in castings can be obtained by using cores. In the present work, a numerical study of heat transfer during the solidification of steel casting with core has been presented. A conjugate conduction based finite element heat transfer analysis of metal, mould and core with interfacial air gap has been performed. The heat transfer through the air gap has been modeled using the coincident node technique. Effect of core materials on solidification times and patterns of square shaped steel castings has been studied. The solidification time and sequence of solidification pattern is different when zircon is used as core material in comparison to silica.